When preparing medicines containing therapeutic proteins, it is often necessary to know the stability indicators such as melting temperature, and/or heat capacity of the protein solution. A known method for determining such thermal properties of a sample is differential scanning calorimetry (DSC).
A typical capillary DSC system comprises two tubes or cells, one for a sample and one for a reference. Each tube is formed into a coil around a hub. The system heats the two cells using heaters located on the respective hub and monitors the temperature gradient between the sample and reference cells, for example using a Peltier device located between the hubs. The temperature difference can be used to drive a feedback loop which drives the heaters to bring the cells back to equilibrium, or alternatively the system can be left to return to equilibrium passively through conduction. The resulting thermogram of thermal gradient versus temperature of the system can be used, for example, to measure the melting temperature or the heat capacity of the protein solution.
Some protein solutions form a gel when a high concentration sample is heated to a sufficiently high temperature. The gel formation temperature can be used to infer the stability of the protein at lower temperatures. However, this tendency to form a gel has previously made it difficult to analyse high concentration protein samples. The gel blocks the sample cell, and is very hard to clean out once it has formed. The higher the concentration of protein in the protein solution, the more difficult it can be to remove all of the gel.
An existing method for removing a gel from a sample cell is to manually clean with fuming nitric acid. Typically, an operator drops the acid into the sample cell using a pipette, and removes any gel waste that is dislodged by the acid. The process may have to be repeated multiple times until the all of the gel has disintegrated, possibly taking several hours to complete. The fuming nitric acid is a health hazardous solution and even short exposure to it could cause permanent damage to the operator.
As a result of the difficulty of cleaning protein gels from the sample cell, conventional DSC systems are typically limited to measuring protein solutions with protein concentrations in the order of 1 mg/ml. However, as therapeutic protein solutions are formulated at higher and higher protein concentrations, it is necessary to measure thermal properties of protein solutions with much higher concentrations, for example 100 mg/ml and over.